Lateral P-N Junctions Based on 2-D Materials

The discovery of graphene marked a turning point in research and interest towards 2 -D materials. Among them, Transition Metal Dichalcogenides (TMDs) and Metal Monochalcogenides (MM) have seen an upturn in interest owing to their versatile properties. Although, they have been studied for many years in bulk form, recent advances in nano-technology enabled new opportunities to study the role of atomically thin materials. In recent years much work has been dedicated to development of their application for the next generation of electronic and optoelectronic devices, and we are witnessing the dawn of the exploration of their properties. In Chapter 1 a brief introduction of highlighted properties of the newly emerged 2 -D materials and their heterostructures is provided. Chapter 2 focuses on field-effect transistor response of few atomic layers of MoSe2, MoTe2 and WSe2. In contrast to previous reports on MoSe2 FETs electrically contacted with Ni, MoSe2 FETs electrically contacted with Ti display ambipolar behavior with current ON to OFF ratios up to 10^6 for both hole and electron channels when applying a small excitation voltage. For both channels the Hall effect indicates Hall mobilities H = 250 cm^2/V.s. Our MoTe2 field-effect transistors are observed to be hole-doped, displaying ON/OFF ratios surpassing 10^6 and typical subthreshold swings of ~140mV per decade. Both field-effect and Hall mobilities indicate maximum values approaching or surpassing 10 cm^2/V.s, which are comparable to figures previously reported for single or bilayer MoS2 and/or for MoSe2 exfoliated onto SiO2 at room temperature and without the use of dielectric engineering. Temperature dependent comparison between field-effect and Hall mobilities in field effect transistors based on few-layered WSe2 exfoliated onto SiO2 is also reported. We observe maximum hole mobilities approaching 350 cm^2/V.s at T = 300 K. The hole Hall mobility reaches a maximum value of 650 cm^2/V.s as T is lowered below ~150 K, indicating that insofar WSe2- based field-effect transistors (FETs) display the largest Hall mobilities among the transition metal dichalcogenides. Chapter 3 evaluates electrostatically gated p-n junctions based on MoSe2 and the photovoltaic response of electrostatically generated p-n junctions composed of approximately 10 atomic layers of MoSe2 stacked onto dielectric h-BN is presented. In addition to ideal diode-like response, we find that these junctions can yield photovoltaic effciencies exceeding 14% under standard solar simulator spectrum with fill factors values of about 70 %. Chapter 4 presents electrical and optical characterization of monolayer and bilayer lateral heterostructures of MoS2-WS2 and MoSe2-WSe2, grown by a one-pot chemical vapor deposition (CVD) synthesis approach, using a single heterogeneous solid source, a newly developed CVD growth method that eliminates the need for the exchange of multiple sources which leads to sample air exposure. The structures show a diode like response which is enhanced under optical illumination. Additionally, bilayer lateral heterostructures exhibit a clear photovoltaic response to optical excitation.